Measurement of trace element concentration distribution, and evaluation of carriers, in semiconductors, and preparation of standard samples

ABSTRACT

A concentration distribution in a planar direction or in a depth-wise direction is measured by irradiating an ion containing an alkali metal element as an ion beam onto a solid surface, detecting a three-atom composite ion comprising the irradiated alkali metal ion, an object element, and a base material element from among particles emitted from the solid surface due to sputtering by mass separation, and displaying an intensity distribution of the three-atom composite ion as a two-dimensional image in the case of the concentration distribution in the planar direction or displaying the intensity of the three-atom composite ion with the sputter time in the case of the concentration distribution in the depth-wise direction. A carrier concentration distribution is obtained by irradiating primary ions to the surface of a semiconductor, into which an electrically conductive impurity is introduced, under conditions such that electrically charge up occurs on the surface of the first semiconductor, sequentially measuring the intensity of secondary ions emitted from the surface and having a specific energy level, during the irradiation time, and calculating the concentration distribution from the carrier concentration corresponding to the intensity of the secondary ions and from an etching quantity of the first semiconductor corresponding to the irradiation time of the primary ions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for measuring the concentrationdistribution of trace elements. More particularly, it relates to amethod of analyzing and evaluating, with a high sensitivity and withhigh resolution, the existence and distribution of trace impuritiescontained in semiconductor materials, metallic materials, ceramicsmaterials and organic materials in both depth-wise and planardirections.

The present invention also relates to an evaluation method for carriersin semiconductors and a method of preparing a standard sample. Speakingin further detail, the present invention relates to a method ofevaluating the carrier concentration and the activation ratio ofcarriers in semiconductors, for example, by the measurement of theintensity of secondary ions, and a method of preparing standard samplesused for this measurement method.

2. Description of the Related Art

Various processes have been attempted, by increasing the impurities insemiconductor materials, metallic materials, ceramic materials, organicmaterials, and so forth, or by deliberately adding impurity orimpurities to them, to improve the material functional properties.However, analysis of trace elements contained in these materials isindispensable for evaluating these processes or evaluating thecharacteristic properties of the materials.

Secondary ion mass spectrometry (SIMS) analysis has been primarily usedin the past as a method of evaluating trace elements existing in thesematerials, and ordinary methods comprise irradiating O₂ ⁺ ions or Cs⁺ions to the surface of a sample and detecting monoatomic ions (M⁺, M⁻)among the secondary ions emitted by sputtering. According to thesemethods, however, it has been extremely difficult to correctly andeasily determine which elements exist in the film because the matrixeffect, under which a detection sensitivity depends on the elementarycomposition of a base material, and the interface effect, under whichthe detection sensitivity changes at an interface of layers, are toopowerfull. Although a method of detecting a two-atom composite ion(CsM⁺) between Cs⁺ and a detection object element (M) has also beenproposed, the sensitivity by this method is not high, and this methodcannot be applied to the analysis of trace elements. Accordingly,development of a method which can analyze trace elements in a solid hasbeen required.

On the other hand, electronic devices such as Josephson junction devicesand MOS ICS use an extremely thin oxide film as a tunnel oxide film or agate oxide film. In these electronic devices, there exists a largenumber of problems with device fabrication relating to the oxide filmsuch as the drop in performance such as withstand voltage or breakdownvoltage of the oxide film resulting from heat-treatment and breakdownafter ion injection, and information on element distribution in theoxide film is extremely important.

Secondary ion mass spectrometry has been employed mainly in the past asan evaluation method of elements existing in the oxide film in thedepth-wise direction, and this method generally irradiates O₂ ⁺ ions orCs⁺ ions to the surface of a sample and detects the monoatomic ions (M⁺,M⁻) among the secondary ions emitted by sputtering. According to thismethod, however, it has been extremely difficult to accurately andeasily determine information of the elements existing in the filmbecause the matrix effect, under which the detection sensitivity dependson the elementary composition of the base material, and the interfaceeffect, under which the detection sensitivity changes at the interfacebetween the layers, are very strong. Recently, the method of detecting atwo-atom composition ion of Cs⁺ and an object element has been proposed,but this method does not clarify the angle of incidence of Cs⁺ onto thesample surface. Accordingly, the reliability of the information thusacquired is not necessarily high.

On the other hand, when an element or elements existing on a fixedsurface are to be determined, it has been customary to subject a sampleto mass analysis without applying any pre-treatment (formation of anoxide film, etc.) to the sample. For this reason, the detectionsensitivity is not sufficiently high, and there are many cases wheretrace elements, which were originally present, cannot be detected.

In a semiconductor device fabrication process, an electricallyconductive impurity such as boron (B) of Group III or phosphorus (P) ofGroup V is doped into a semiconductor substrate by ion implantation orgas diffusion so as to form a P-type layer or an N-type layer and toobtain transistors, and so forth. To obtain the desired characteristics,control of a carrier concentration distribution is necessary. However,since the concentration distribution and the activation ratio of theconductive impurity change with the introduction method and with theheat-treatment condition, it is very important to correctly detect thecarrier concentration distribution.

Spreading resistance analysis (SRA) is generally used at present as anevaluation method of the carrier concentration distribution in thesemiconductors. This spreading resistance analysis is the method whichbrings two or four probes into contact with a sample which is subjectedto oblique polish, to measure a resistance value, and converts theresistance value of the sample to the carrier concentration from therelationship of correspondence between the resistance value of astandard sample and a known carrier concentration. To obtain aconcentration distribution in the depth-wise direction, further, theprobes are sequentially moved on the polished surface in the depth-wisedirection, and the resistance value corresponding to the depth ismeasured.

According to the spreading resistance analysis according to the priorart described above, however, the resistance value to be measured isaffected by the crystalline plane orientation. Therefore, when thesample to be measured is polycrystalline, an error of several percentcan occur in the measured resistance value. Further, resolution in thedepth-wise direction is affected by the accuracy of the obliquepolishing. Accordingly, the depth resolution is about 10 nm and animprovement in this resolution has been desired.

Further, secondary ion mass spectrometry (SIMS) is known as a method ofmeasuring the quantity of conductive impurities. Generally, this methodcomprises bombarding oxygen ions, for example, as the primary ions tothe sample to be measured, measuring the intensity of the secondary ionsemitted from the sample, and specifying the quantity of the conductiveimpurity of the sample from the relationship between the intensity ofthe secondary ions of the standard sample having a known quantity of theconductive impurity and the quantity of the known conductive impurity.However, according to this method, an error occurs in the convertedquantity of the conductive impurity if the standard sample iselectrically charged and, eventually, an error occurs in the measuredquantity of the conductive impurity. Though this method can measure theconcentration distribution of the conductive impurity, it cannot measurethe carrier concentration distribution.

SUMMARY OF THE INVENTION

The present invention is directed to make a contribution to theevaluation of functional properties of materials by effecting ahigh-sensitivity analysis and a high-resolution analysis of traceimpurities contained in semiconductor materials, metallic materials,ceramic materials and organic materials in both depth-wise and planardirections.

The present invention is further directed to make a contribution to theevaluation of characteristics of electronic devices by accuratelyevaluating a concentration distribution of elements existing in an oxidefilm, in a depth-wise direction, existing on the surface of, and betweenlayers of, materials used for the electronic devices.

The present invention is further directed to determine with a highsensitivity the trace elements existing on the fixed surface.

The present invention is further directed to provide an evaluationmethod of carriers in semiconductors capable of improving the accuracyof carrier concentration distribution and depth resolution measurements,and a method of preparating a standard sample.

To accomplish the objects described above, the first invention (claim 1)of the present application provides a method of measuring theconcentration distribution of trace elements in a solid in a planardirection and/or in a depth-wise direction, which comprises irradiatingions containing alkali metal elements such as Li, Cs, etc., as an ionbeam onto a solid surface, detecting a three-atom composite ionconsisting of an irradiated alkali metal ion, an object element, and abase material element, from among the particles emitted from the solidsurface by sputtering, by mass separation, and measuring a concentrationdistribution, in the planar direction, by displaying an intensitydistribution of the three-atom composite ion as a two-dimensional image,or measuring the concentration, in the depth-wise direction, bydisplaying the intensity of the three-atom composite ion with thesputtering time.

The first embodiment of the second invention of the present applicationrelates to a method of measuring a concentration distribution of traceelements existing in a solid in a planar direction and/or a depth-wisedirection, which comprises irradiating an oxygen beam to a solid surfaceand simultaneously irradiating an ion beam containing an alkali metalelement such as Li, Cs, etc., as an ion beam at an angle of incidence offrom 30° to 60° with respect to a normal direction of the sample so asto carry out sputtering, while forming an oxide on the solid surface,detecting a two-atom composite ion comprising the alkali metal ion andthe object element by mass separation from the particles emitted fromthe solid surface, and then measuring the concentration distribution ina planar direction by displaying the intensity of the two-atom compositeion as a two-dimensional image, or measuring the concentrationdistribution in the depth-wise direction by displaying the intensity ofthe two-atom composite ion with the sputtering time.

The second embodiment of the second invention of the present applicationrelates to a method of measuring a concentration distribution of traceelements existing in a solid in a planar direction and/or a depth-wisedirection, which comprises alternately repeating irradiation of anoxygen ion beam and irradiation of an ion beam containing an alkalimetal element such as Li, Cs, etc., (at an angle of incidence of from30° to 60° with respect to the normal of the sample), carrying outsputtering while forming an oxide surface on the solid surface,detecting a two-atom composite ion comprising the alkali metal ion andan object ion by mass separation from among the particles emitted fromthe solid surface, and then measuring the concentration distribution inthe planar direction by displaying the intensity of the two-atomcomposite ion as a two-dimensional image, or measuring the concentrationdistribution in the depth-wise direction by displaying the intensity ofthe two atom composite ion with respect to the sputter time.

Further, the third invention (Example 4) according to the presentapplication relates to a method of measuring a concentrationdistribution of a trace element existing in an oxide film in adepth-wise direction, which comprises irradiating an alkali metal ionsuch as Cs⁺, Li⁺, etc., onto a solid surface or a solid sample in whichan oxide film exists between the layers thereof, at an angle ofincidence of from 60° to 90° with respect to the direction of the normalof a sample, detecting a two-atom composite ion comprising theirradiated alkali ion and an object element (M) by mass separation fromthe secondary ions emitted by sputtering, and measuring theconcentration distribution in the depth-wise direction by displaying theintensity of the two-atom composite ion with respect to the sputtertime.

Further, the fourth invention (Example 5) of the present applicationrelates to a method of detecting an element existing on a solid surface,which comprises forming an extremely thin oxide film on the solidsurface, irradiating an ion beam containing an alkali metal element suchas Li, Cs, etc., at an angle of incidence of 30° to 60° with respect tothe normal of the sample, detecting a two-atom composite ion comprisingthe irradiated alkali metal ion and the element existing on the solidsurface and mass-analyzing the same.

The fifth invention (Examples 6, 8) according to the present applicationprovides an evaluation method of a carrier inside a semiconductor whichcomprises irradiating a primary ion to the surface of a firstsemiconductor into which an electrically conductive impurity isintroduced, under a condition such that electrical charge up takes placeon the surface, sequentially measuring the intensity of a secondary ionemitted from the surface and having specific energy, with the passage ofthe irradiation time of the primary ion and obtaining a carrierconcentration distribution in the semiconductor in the depth-wisedirection from the carrier concentration corresponding to the intensityof the secondary ion and from an etching quantity of the firstsemiconductor corresponding to the irradiation time of the primary ion.

The sixth invention (Example 7) according to the present applicationprovides a method of evaluating a carrier inside a semiconductor whichcomprises irradiating a primary ion onto the surface of a secondsemiconductor into which an electrically conductive impurity isintroduced under the same condition where an electrically conductiveimpurity is introduced into a first semiconductor, under a conditionsuch that electrically charge up does not take place on the surface,measuring the intensity of the secondary ion emitted from the surface soas to acquire the concentration distribution of the conductive impurityin the second semiconductor in the depth-wise direction, and acquiringan activation ratio using the concentration distribution of theconductive impurity in the second semiconductor and the carrierconcentration distribution in the first semiconductor.

The seventh invention (Examples 6, 7) according to the presentapplication provides a method of preparating a standard sample used forconverting the intensity of secondary ions emitted from a semiconductorby the irradiation of a primary ion to the semiconductor having anelectrically conductive impurity implanted thereto, to a carrierconcentration or a conductive impurity concentration, which comprisesintroducing in advance an electrically conductive impurity into asemiconductor to serve as a standard sample so as to lower theresistivity of the conductor, and introducing an electrically conductiveimpurity having a known quantity into the semiconductor described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a concentration distribution of O in Si in adepth-wise direction;

FIG. 2 is a graph showing a distribution of O and H in anNbN/Nb/Al/Nb/Si substrate in a depth-wise direction;

FIG. 3 shows a secondary ion image on an Si substrate surface;

FIG. 4 is a graph showing a concentration distribution of O and Si in adepth-wise direction by irradiation with Cs⁺ ;

FIG. 5 is a graph showing a concentration distribution of O and Si in adepth-wise direction by alternate irradiation with O₂ ⁺ and Cs⁺ ;

FIG. 6 is a graph showing a concentration distribution of O and Si in adepth-wise direction by irradiation with Cs⁺ ;

FIG. 7 is a graph showing a concentration distribution of O and Si in adepth-wise direction by irradiation with Cs⁺ ;

FIG. 8 is a graph showing a concentration distribution of O and Si in adepth-wise direction (at an angle of incidence of Cs⁺ of 30°);

FIG. 9 is a graph showing a concentration distribution of O and Si in adepth-wise direction (at an angle of incidence of Cs⁺ of 45°);

FIG. 10 is a graph showing a concentration distribution of O and Si in adepth-wise direction (at an angle of incidence of Cs⁺ of 60°);

FIG. 11 is a graph showing a concentration distribution of O and Si in adepth-wise direction (at an angle of incidence of Cs⁺ of 80°);

FIG. 12 is a graph showing a measurement result after SiO₂ is formed onthe Si surface;

FIG. 13 is a graph showing an analysis result according to the fourthembodiment;

FIG. 14 is a graph showing an analysis result according to the priorart;

FIGS. 15(a) and (b) are explanatory views each useful for explaining ameasurement method of a carrier concentration;

FIGS. 16(a) and (b) are graphs each showing a measurement result of aconcentration distribution of a carrier and boron;

FIG. 17 is a graph showing a measurement result of a carrierconcentration distribution by a spreading resistance analysis method;

FIG. 18 is a diagram showing the relationship between a shift quantityof peak energy and a thickness of an oxide film of a base;

FIGS. 19(a) to (d) are sectional views, each useful for explaining apreparation method of a standard sample;

FIGS. 20(a) to (d) are explanatory views each useful for explaining therelationship between a shift quantity of peak energy and an injectionquantity of an electrically conductive impurity;

FIGS. 21(a) and (b) are explanatory views each useful for explaining ameasurement method of an electrically conductive impurity of a standardsample;

FIGS. 22(a) to (c) are explanatory views each useful for explaining themeasurement result of an impurity concentration of the standard sample;

FIG. 23 is an explanatory view useful for explaining a secondary ionmass spectrometer; and

FIG. 24 is a perspective view useful for explaining a spreadingresistance analysis method according to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first invention of the present invention, a method which sets anangle of incidence of an irradiation ion beam to 30° to 90° with respectto the direction of the normal of a sample so as to improve thesensitivity (claim 2) is preferred. Further, a method which sets theangle of incidence of the irradiation ion beam to 60° to 90° withrespect to the direction of the normal of the sample is preferred so asto improve the resolution in a depth-wise direction (claim 3).

To improve resolution in a planar direction, a method which sets theangle of incidence of the irradiation ion beam to 0° to 30° to thedirection of the normal of the sample (claim 4) is preferred.

In other words, the ion beam is scanned so as to catch an image of thesecondary ion, but when the angle of incidence is tilted, the beam shapebecomes elliptic on the sample surface. To obtain an image having highresolution, the beam shape on the sample surface must be made sharp, anda suitable inclination is within 30°.

To analyze trace elements contained in an oxide film with a highsensitivity, the angle of incidence of an irradiation alkali metal ionbeam is preferably set to 60° to 90° to a solid surface having the oxidefilm on the surface or between layers thereof (claim 5).

As described above, according to the first invention, the angle ofincidence of the ion beam incident into the solid is suitably selectedin accordance with the respective objects such as high sensitivityanalysis, mass spectrometry with high resolution, analysis with highplanar resolution, analysis of the oxide film, and so forth.

According to the means described above, the problems of the matrixeffect and the interface effect that have been the problems with theprior art methods can be eliminated, and high-sensitivity,high-resolution analysis becomes possible in accordance with theintended object.

According to the first embodiment of the second invention, the oxygenion and the alkali metal ion are simultaneously irradiated under thecondition such that the injection depth of the oxygen ion is greaterthan that of the alkali metal ion. This condition can be determined byconsidering the acceleration energy and the angle of incidence of theoxygen ion.

Alternatively, this condition can be obtained through the accelerationenergy of the alkali metal ion (or even by limiting the angle ofincidence of the alkali metal ion to 30° to 60°).

According to "Secondary Ion Mass Spectrometry" by Robert G. Wilson, FredA. Stevie and Charles W. Magee (John Wiley & Sons Publication, 1989), p1.2-1, the injection depths of O₂ ⁺, Ar⁺ and Cs⁺ are given by thefollowing formulas I to III, respectively:

    O.sub.2.sup.+ : R=2.15E cos θ                        (I)

    Ar.sup.+ : R=1.622E.sup.0.84 cos θ                   (II)

    Cs.sup.+ : R=1.838E.sup.0.64 cos θ                   (III)

(where R is a penetration depth (nm), θ is an angle of incidence fromnormal, and E is primary energy (keV)). Accordingly, condition settingis easy.

In the second embodiment, the thickness of the oxide film formed on thesample surface is preferably from 2 to 5 nm.

As described above, the second invention of the present applicationprovides a method which forms an extremely thin oxide film having athickness of 2 to 5 nm on the solid surface by vacuum deposition, etc.,and irradiates an ion beam containing alkali metal such as Li, Cs, etc.,in the presence of this ultra-thin oxide film at the angle of incidenceof 30 to 60 degrees. During detection, this method catches two-atomcomposite ions comprising the alkali metal ion and the object elementfrom among the particles emitted from the solid surface due tosputtering, and effects mass analysis of the composite ions so as toevaluate the concentration distribution of the detection object elementin both the depth-wise direction and the planar direction.

According to the means described above, the matrix effect and theinterface effect that have been the problems with the prior art methodscan be eliminated and high-sensitivity analysis becomes possible.

The third method of the present invention conducts mass analysis for thesolid sample existing on the surface or between the layers by the methoddescribed above, and can accurately measure the concentrationdistribution of the elements existing in the film in the depth-wisedirection. Therefore, the third method can make contribution to theevaluation of the characteristics of electronic devices.

The fourth invention (Example 5) forms in advance an oxide film on asample surface and then irradiates ions containing an alkali metalelement at a predetermined angle. In this way, the fourth invention canenhance the sensitivity and determines the trace elements existing onthe solid surface.

An evaluation method of a carrier of a semiconductor according to thepresent invention (Examples 6, 8) irradiates primary ions on the surfaceof the semiconductor under conditions such that electrical charge upoccurs on the surface of the semiconductor, sequentially measures theintensity of the secondary ions jumping out from the semiconductor andhaving specific energy with the passage of the irradiation time of theprimary ions, and acquires the carrier concentration distribution in thesemiconductor in the depth-wise direction from a carrier concentrationcorresponding to the intensity of the secondary ions and an etchingquantity of the semiconductor corresponding to the irradiation time ofthe primary ions.

The present invention utilizes the property that the charge quantity atthe surface of the semiconductor depends on the resistivity of thesemiconductor surface, that is, on the carrier concentration of thesemiconductor surface. In other words, the present invention irradiatesthe primary ions under the condition such that the semiconductor surfaceis electrically charged up, and controls the intensity of the secondaryions having specific energy. Now, the energy distribution of thesecondary ions irradiated depends on the charge quantity and shifts tothe high energy side or to the low energy side. For this reason, acertain specific correlationship develops between the carrierconcentration of the semiconductor surface and the shift quantity of theenergy distribution of the secondary ion intensity. Since the shape ofthe energy distribution of the secondary ion intensity is hardlyaffected by the shift in the energy distribution, the intensity of thesecondary ions corresponding to the carrier concentration can bemeasured by measuring the intensity of the secondary ions havingspecific energy.

Accordingly, the carrier concentration distribution in the semiconductorin the depth-wise direction can be obtained by converting the intensityof the secondary ions measured with the passage of the irradiation timeof the primary ions to the carrier concentration.

Since the etching quantity due to the irradiation of the primary ions isused to detect the position in the depth-wise direction, higher accuracycan be obtained than the spreading resistance analysis method using thepolishing method according to the prior art.

A standard sample having a known quantity of a known impurityconcentration is used for the conversion from the secondary ionintensity to the carrier concentration. In this case, if the energydistribution of the secondary ion intensity shifts because the standardsample is electrically charged up, an error occurs in the convertedcarrier concentration. Accordingly, it is necessary to prevent thestandard sample from being electrically charged up.

In the preparation method of the standard sample according to thepresent invention, another conductive impurity is in advance introducedbefore the introduction of the known quantity of the conductive impurityinto the semiconductor as the standard sample so as to lower theresistivity of the semiconductor as the standard sample.

Accordingly, even when the concentration of the known quantity of theconductive impurity to be introduced into the semiconductor is low,electrically charge up does not occur at the time of the irradiation ofthe primary ion, and the intensity of the secondary ions can be measuredwith a high level of accuracy. For this reason, the known quantity ofthe conductive impurity in the standard sample is accurately acquired.

Having the construction described above, the present invention cananalyze with a high sensitivity the trace elements contained insemiconductor materials, metallic materials, ceramic materials and inorganic materials in both the depth-wise direction and the planardirection, and can make a contribution to the evaluation of functionalproperties of these materials. Further, the present invention canaccurately measure the elements existing in the oxide film used in theelectronic devices in the depth-wise direction and can thus make acontribution to the evaluation of the characteristics of electronicdevices.

The evaluation method of the carrier of the semiconductor according tothe present invention utilizes the facts that when irradiated with theprimary ions at a specific angle of incidence, the polysilicon film 3(FIG. 15) is charged electrically and that the carrier concentration andthe charge quantity of the polysilicon film 3 have the relation ofmutual dependence, and provides correlationship between the carrierconcentration and the shift quantity of the energy distribution of thesecondary ion intensity.

Accordingly, the carrier concentration distribution in the polysiliconfilm in the depth-wise direction can be obtained by measuring theintensity of the secondary ions having specific energy with the passageof the irradiation time of the primary ions and further converting it tothe carrier concentration.

Since the present invention uses the etching quantity due to theirradiation of the primary ions for determining the position in thedepth-wise direction, the present invention provides higher accuracythan the spreading resistance analysis method using the polishingmethod.

Furthermore, in the preparation method of the standard sample accordingto the present invention, another conductive impurity is in advanceintroduced before the introduction of the known quantity of theconductive impurity so as to lower the resistivity of the substrate ofthe standard sample.

Accordingly, even when the concentration of the known quantity of theconductive impurity to be introduced into the standard sample is low,electrical charge up does not occur at the time of the irradiation ofthe primary ions, so that the intensity of the secondary ions can bemeasured precisely and, eventually, the concentration of the knownquantity of the conductive impurity can be obtained precisely.

Hereinafter, the present invention will be explained in further detailwith reference to Examples thereof, but the invention is not naturallylimited thereto.

EXAMPLE 1 Relating to the First Invention (Claim 1)

A sample was produced by ion-implanting ¹⁸ O⁺ into an Si substrate at100 keV in a dose of 5×10¹⁴ atoms/cm². Next, Cs⁺ ion beam was irradiatedto this sample surface at an angle of incidence of 54° so as to increaseits sensitivity. The result is shown in FIG. 1. It can be understoodfrom FIG. 1 that detection of CsSiO⁺ (Mass 177.0 amu) had a much highersensitivity than the detection of CsO⁺ (Mass 148.9 amu) according to theprior art method. FIG. 2 shows the result of measurement of theconcentration distributions of O and H in the depth-wise direction inthe structure of an NbN/Nb/Al/Nb/Si substrate, and Cs⁺ ion beam wasirradiated at an angle of incidence of 72° in order to obtain highresolution in the depth-wise direction. Since the detection of CsNbO⁺(Mass 241.8 amu), CsNbH⁺ (Mass 226.8 amu) had a higher sensitivity thanthe detection of CsO⁺, CsH⁺ (Mass 133.9 amu) according to the prior artand the distribution on the interface between the layers could beacquired more distinctively, resolution in the depth-wise direction,too, was higher.

Next, the detection limit of ¹⁸ O⁺ in Si determined from the sampleobtained by ¹⁸ O⁺ ion implantation into the Si substrate is shown inTable 1.

                  TABLE 1                                                         ______________________________________                                        Detection limit of O in Si                                                    angle of incidence                                                            (°)     detection limit (cm.sup.-3)                                    ______________________________________                                        36             7.0 × 10.sup.17                                          45             7.0 × 10.sup.16                                          54             5.0 × 10.sup.16                                          60             6.0 × 10.sup.16                                          72             2.5 × 10.sup.17                                          80             5.0 × 10.sup.17                                          ______________________________________                                    

As is obvious from the result tabulated in Table 1, it was preferred toset the incident angle of the irradiation ion beam to 30° to 90°,particularly 45° to 60°, with respect to the normal of the sample, inorder to improve the sensitivity.

EXAMPLE 2 The First Invention (Claim 1)

CsSiO⁺ was detected in foreign matter on the surface of the Si substrateby the use of Cs⁺ ion beam as the primary ions. A Cs⁺ ion beam having adiameter of about 10 μm was scanned on the substrate surface in an X-Ydirection, and the intensity of CsSiO⁺ was displayed as a secondary ionimage with the x-y position. FIG. 3 shows the resulting secondary ionimage. White portions indicate that the intensity of CsSiO⁺ was high andthat O existed in the foreign matter.

EXAMPLE 3 Relating to the Second Invention

Next, an example of the invention wherein the irradiation of the oxygenion beam and the alkali metal element ion beam was alternately repeatedwill be illustrated.

FIGS. 4 and 5 show the results of measurement of the concentrationdistribution of O and Si in the depth-wise direction using a sampleobtained by forming a 5 nm-thick SiO₂ on an Si (100) substrate bythermal oxidation. FIG. 4 shows a concentration distribution obtained byusing only an ion beam containing an alkali metal (Cs⁺ beam) (accordingto the prior art), and FIG. 5 shows the concentration distributionobtained by the alternate irradiation of the oxygen ion beam and the ionbeam containing the alkali metal (Cs⁺ beam). The angle of incidence ofthe Cs⁺ beam was 30° in FIG. 4 and that of Ce⁺ ion beam and O⁺ ion beamwas 60° respectively in FIG. 5, and a two-atom composite ion comprisingCs and the object element was detected. Whereas the intensity of CsSi⁺in the Si substrate was 1×10³ count in FIG. 4, it was 5×10³ count inFIG. 5, and the sensitivity could be improved five times. Accordingly,when any trace elements existed in the Si substrate, the presentinvention could detect them with a sensitivity which was by five timeshigher than the prior art method.

In this second invention, the thickness of the oxide film to be formedon the sample surface at the time of analysis was preferably from 2 to 5nm.

FIG. 7 shows a concentration distribution of Si in the depth-wisedirection of a sample produced in Example 3 wherein the thickness ofSiO₂ was 10 nm. FIG. 6 shows the concentration distribution of O and Siin the depth-wise direction in the sample of FIG. 7 wherein thethickness of SiO₂ was changed to 2 nm by HF etching. The change of theintensity of CsSi⁺ shown in FIG. 4 could also be observed in FIGS. 6 and7, too. However, in FIG. 7, the intensity of CsSi⁺ gradually droppedtowards the SiO₂ /Si substrate interface in FIG. 7. The thickness atwhich the effect of the oxide film could be observed was up to about 5nm which was a half of 10 nm. When the thickness of the oxide film wassmaller than 2 nm, the concentration of the alkali metal ion to beirradiated in common in either case did not reach a predetermined level(the state where sputtering was not steady), the effect of the presentmethod was believed low. Accordingly, the thickness of the oxide film tobe formed on the sample surface during analysis according to the presentinvention was preferably limited to from 2 to 5 nm.

EXAMPLE 4 Relating to the Third Invention

FIGS. 8 to 11 show the results when the alkali metal ion Cs⁺ wasirradiated to a sample obtained by forming SiO₂ to a thickness of 10 nmon the Si substrate, by changing the angle of incidence of Cs⁺ ion beamto the sample surface to 30°, 45°, 60° and 80°, and two-atom compositeion CsSi⁺ and CsO⁺ was detected. In FIGS. 8 and 9, the secondary ionintensity of CsSi⁺ and CsO⁺ in SiO₂ decreased from the sample surfacetowards the SiO₂ /Si substrate interface and the profile of CsSi⁺ in thedepth-wise direction was discontinuous on the SiO₂ /Si substrateinterface. On the other hand, in FIGS. 9 to 11, particularly in FIGS. 10and 11, the intensity of the secondary ions of CsSi⁺ and CsO⁺ in SiO₂was substantially constant and no discontinuous portions existed in bothof them in the depth-wise direction. It could be understood from theseresults that the concentration distribution of the element existing inthe oxide film in the depth-wise direction could be measured accuratelyby irradiating Cs⁺ at an angle of incidence ranging from 60° to 90° tothe sample surface and detecting CsM⁺ as shown in FIGS. 10 and 11.

EXAMPLE 5 Relating to the Fourth Invention

I) After a 2 nm-thick SiO₂ film was formed on the Si substrate surface,Cs⁺ was irradiated at an angle of incidence of 30°. FIG. 12 shows theresult of detection of the ions (CsSi⁺ and CsO⁺) emitted from the solidsurface due to the sputtering. It can be understood from FIG. 12 thatthe CsSi⁺ intensity on the sample surface increased due to the influenceof the oxide film.

II) FIGS. 13 and 14 show mass spectrograms of the sample obtained bydoping B into the Si substrate. FIG. 14 shows the result according tothe prior art method and FIG. 13 shows the result according to thepresent method (the method which first formed the thin oxide film on thesample surface and then made an analysis). Whereas B (boron) wasdetected as CsB⁺ in FIG. 13, B could not be detected in FIG. 14.

EXAMPLE 6 Preparation of Standard Sample According to the Embodiment ofthe Present Invention

FIGS. 20(b) to (d) are sectional views useful for explaining thepreparation method of a testpiece used for an electrical charge up test.

FIG. 20(b) shows the state where a 50 nm-thick silicon dioxide film 12was first formed on an Si substrate 11 by thermal oxidation and then a400 nm-thick polysilicon film 13 was formed on the silicon dioxide film12. By the way, the polysilicon film 13 was electrically insulated onthis substrate so that charging up could easily take place particularlyin or on the poly-silicon film 13.

Six of the substrates described above were prepared. As shown in FIG.20(c), ion implantation was carried out into these six substrates underthe following conditions, respectively, and six kinds of testpieces wereproduced.

(a) electrically conductive impurity (pre-implantated element: boron(B): three kinds of doses, i.e. 1×10¹², 1×10 and 1×10¹⁴ cm⁻²

(b) electrically conductive impurity (pre-injection element: phosphorus(P): three kinds of doses, i.e. 1×10¹², 1×10¹³ and 1×10¹⁴ cm⁻²

Subsequently, each of these substrates was heat-treated at 900° C. for60 minutes under a nitrogen atmosphere as shown in FIG. 20(d) so as toactivate the implanted conductive impurity. In this way, the testpieceswere completed.

Next, one of the testpieces was set into a secondary ion massspectrometer as shown in FIG. 23. Then, oxygen as the primary ion wasirradiated to the surface of the testpiece as shown in FIG. 21(b). Atthis time, the secondary ions of the conductive impurity having variousenergy levels jumped out from the surface of the testpiece, and energyof the secondary ions to be detected was changed by regulating an offsetvoltage to be applied to the sample stage. In this way, the secondaryions having various levels of energy were sequentially measured, and anenergy distribution of the intensity of the secondary ions was acquired.At this time, peak energy of each testpiece was measured by settingenergy at the peak position of the secondary ion intensity emitted fromthe sample, which was not electrically charged (hereinafter referred toas the "peak energy"), Ep0, to zero, and using this as the reference.The result is shown in the graph of FIG. 20(a).

According to this result, when the pre-implantation element was boron, ashift of peak energy Ep1 of about 1.5 eV existed at the dose of 1×10¹²cm⁻² and 1×10¹³ cm⁻². In contrast, peak energy Ep0 became zero at a doseof 1×10¹⁴ cm⁻².

On the other hand, when the pre-implantation element was phosphorus,peak energy did not become zero at any dose (FIG. 20a). In other words,it was understood that the testpiece was always charged electrically andphosphorus was not suitable as a conductive impurity to be implanted inadvance.

Accordingly, it became necessary to dope in advance boron in a dose ofat least about 1×10¹⁴ cm⁻² into the standard sample.

EXAMPLE 7 Preparation of a Standard Sample Used for the Evaluation ofCarrier Concentration According to the Present Invention

Next, the preparation method of the standard sample according to anembodiment of the invention and a method of converting the intensity ofthe secondary ions to the concentration of the conductive impurity usingthe standard sample will be explained.

First of all, a substrate having a polysilicon film 13a, into whichboron (the pre-implantation element) in an amount of about 2×10¹⁸ cm⁻³(substantially corresponding to 1×10¹⁴ cm⁻² when calculated into a dose)was in advance introduced, was prepared in the same way as in FIGS.20(b) to (d) (FIGS. 19(a) to (c)).

Next, boron ions were implanted into the substrate under the same ionimplantation conditions as those for that boron (the same dose andacceleration energy). In this way, the preparation of the standardsample, into which a known quantity of boron was introduced into thepolysilicon film 13b, was completed as shown in FIG. 19(d).

Next, the standard sample was set to the secondary ion mass spectrographshown in FIG. 23. Before the measurement was started, the voltage of thesample stage was in advance regulated to peak energy Ep0 of thesecondary ion intensity emitted from a sample not charged electrically.

Subsequently, oxygen as the primary ions was irradiated to the surfaceof the standard sample. At this time, secondary ions having variouslevels of energy jumped out from the surface of the standard sample, butonly the secondary ions having specific energy Ep0 were detected by theimpression or application of the voltage to the sample stage. After thepassage of a predetermined time, the intensity of the secondary ions wasmeasured. The intensity of the secondary ions was sequentially measuredwith the passage of the irradiation time of the primary ions by such ameasurement method. In this case, since boron was in advance injectedinto the standard sample and its resistivity was lowered, charging updid not occur during the measurement. Accordingly, since the shift ofthe energy distribution of the secondary ion intensity did not occur,the peak value of the secondary ion intensity distribution could bemeasured at all the measurement points.

Next, the etching quantity corresponding to the irradiation time wasplotted on the abscissa and the intensity of the secondary ions, on theordinate. In this way, the distribution of the intensity Is(x) of thesecondary ions in the depth-wise direction could be obtained.

A proportional coefficient A could be obtained in accordance with thefollowing equation on the basis of the graph obtained in the mannerdescribed above:

    φ=A·ΣIs(x)dx                            (1)

where φ: total dose=dose×dose time, A: proportional coefficient.

Using this proportional coefficient A, the concentration N(x) of thecarrier and the conductive impurity at a certain depth x could beobtained in accordance with the following equation (2):

    N(x)=A·Is(x)                                      (2)

The concentration N(x) of the carrier and the conductive impurity at acertain depth x can be obtained by using the equation (2). Accordingly,the concentration distribution of the carrier and the conductiveimpurity can be determined in the depth-wise direction by effectingconversion for all of Is(x) measured with the passage of the irradiationtime.

According to the preparation method of the standard sample of thepresent invention, the resistivity of the poly-silicon film 13a waslowered by introducing, in advance, boron before the known quantity ofboron was introduced into the poly-silicon film 13. For this reason,electrical charge up did not occur in the poly-silicon film 13b at thetime of the measurement of the intensity of the secondary ions after theknown quantity of boron was introduced into the poly-silicon film 13a.In other words, since the shift of the energy distribution of thesecondary ion intensity did not occur, the peak value of the secondaryion intensity could be measured at all the measurement points.

Because the concentration of boron, which was introduced in advance, wasset to be lower than the concentration of the known quantity of theboron concentration, which was to be measured, the known quantity of theboron concentration to be measured could be measured without beingaffected by the concentration of boron introduced in advance.

Accordingly, the proportional coefficient A, at a high level ofaccuracy, could be acquired and, eventually, the concentrationdistribution of the carrier and the conductive impurity in thedepth-wise direction could be acquired with a high level accuracy forthe object material to be measured.

Incidentally, boron could be used as the conductive impurity introducedin advance, but a conductive impurity different from the known quantityof the conductive impurity to be measured could also be used.

COMPARATIVE EXAMPLE 1

FIGS. 22(b) and (c) are sectional views useful for explaining thepreparation method of the standard sample according to a ComparativeExample, and fluorine (¹⁹ F) was introduced as an impurity into thestandard sample. Unlike the standard sample of the Examples of thepresent invention, however, the conductive impurity was not introducedin advance into the poly-silicon film and fluorine (¹⁹ F) was introducedunder the state where the resistivity was high.

FIG. 22(a) shows the result of measurement by the secondary ion massspectrometry of the concentration distribution of fluorine (¹⁹ F) in thestandard sample.

In FIG. 22(a), the fluorine concentration in the poly-silicon film 23and the silicon dioxide film 22 indicated a higher concentration thanthe true concentration distribution. This was presumably because theenergy distribution of the secondary ion intensity shifted as shown inFIG. 21(a) and a measurement error occurred.

EXAMPLE 8 Measurement of Carrier Concentration Distribution in aSemiconductor According to the Invention

FIG. 15(b) is a sectional view useful for explaining a test body usedfor the evaluation of the carrier concentration according to anembodiment of the present invention.

First of all, a 50 nm-thick silicon dioxide film 2 was formed on an Sisubstrate 1 by thermal oxidation, and then a 400 nm-thick poly-siliconfilm 3 was formed on the silicon dioxide film 2. Incidentally, thepoly-silicon film 3 was electrically insulated from this substrate sothat electrical charge up could easily occur particularly in or on thepoly-silicon film 3.

Next, boron (B) was introduced as an electrically conductive impurity ina dose of 1×10¹⁵ cm⁻² into the poly-silicon film 3 by ion implantation.

Each sample was heat-treated at 900° C. for 60 minutes under a nitrogenatmosphere so as to activate the injected conductive impurity. In thisway, the test body was completed.

Next, the test body was placed in the secondary ion mass spectrographshown in FIG. 23. Before the measurement was started, the voltage of thesample stage was in advance regulated to peak energy Ep0 of thesecondary ion intensity emitted from the uncharged sample.

Subsequently, oxygen, as the primary ion, was irradiated to the surfaceof the poly-silicon film 3 under the condition where charge occurred onthe surface of the poly-silicon film 3 at all the measurement points oftime, that is, at an angle of incidence of Θ=22° to the normal of thesurface of the poly-silicon film 3. The secondary ions of boron havingvarious energy levels jumped out from the surface of the poly-siliconfilm 3, but only the secondary ions having specific energy Ep0 weredetected by the impression or application of the voltage on the samplestage. After the passage of a predetermined time (t), the intensity I(t)of the secondary ions was measured.

Since the primary ions were irradiated under the condition whereelectrical charge up took place, that is, at an angle of incidence ofΘ=22°, the poly-silicon film 3 was electrically charged while dependingon the carrier concentration. The energy distribution of the secondaryion intensity I(t) shifted to the high energy side or to the low energyside depending on the charge up quantity as shown in FIG. 15(a). At thistime, the shape of the energy distribution was hardly affected by the 10shift of the energy distribution and moreover, only the secondary ionshaving specific energy Ep0 were detected. Accordingly, the intensityI(t) of the secondary ions corresponding to the carrier concentrationcould be measured.

The intensity I(t) of the secondary ion was sequentially measured duringthe irradiation time by such a measurement method. The results wereshown in the graph of FIG. 16(a). The axis of abscissa represents theirradiation time and the ordinate represents the secondary ionintensity.

After a series of secondary ion intensity values I(t) were measured, theetching quantity corresponding to the irradiation time was derived, andthe intensity I(x) of the secondary ions corresponding to the depth wasacquired.

Next, the intensity I(x) of the secondary ions measured was converted tothe carrier concentration C(x) in accordance with the equation (2) usingthe proportional coefficient A determined in advance from the standardsample described above. In this way, the carrier concentrationdistribution in the depth-wise direction, which corresponds to FIG.16(a), could be obtained.

For reference, FIG. 17 shows the carrier concentration distribution ofthe same body which was obtained by the spreading resistance analysismethod. The present invention could obtain more precise concentrationdistribution values.

As described above, the measurement method of the carrier concentrationdistribution in the embodiment of the present invention utilized theproperties that the poly-silicon film 3 was electrically charged by theirradiation of the primary ions at a specific angle of incidence andthat the carrier concentration and the charge up quantity of thepoly-silicon film 3 had a mutual relationship, and provided acorrelation between the carrier concentration and the shift quantity ofthe energy distribution of the secondary ion intensity. Since the shapeof the energy distribution of the secondary ion intensity was hardlyaffected by the shift of the energy distribution, the intensity of thesecondary ions corresponding to the carrier concentration could bemeasured by measuring the intensity of the secondary ions having aspecific energy level.

Accordingly, the carrier concentration distribution in the poly-siliconfilm 3 in the depth-wise direction could be obtained by converting theintensity of the secondary ions measured during the irradiation time ofthe primary ions to the carrier concentration.

Since the etching quantity due to the irradiation of the primary ionswas used to obtain the position in the depth-wise direction, accuracycould be made higher than the spreading resistance analysis method usingthe polishing method according to the prior art.

In the embodiment described above, the primary ions were irradiated tothe surface of the sample under the condition where electrical charge upoccurred on the surface of the poly-silicon film 3, that is, at an angleof incidence of θ=22° to the normal of the surface of the poly-siliconfilm 3. However, the angle of incidence θ could be set to 30° or belowas the condition where charge up occurred on the surface of the sample.

The thickness of the silicon dioxide film 2 as the base of thepoly-silicon film 2 was 50 nm in the embodiment described above, but thethickness could also be set to a suitable film thickness as shown in thedrawings. FIG. 18 shows the result of the peak shift energy of thesecondary ion mass spectrograph of the poly-silicon film 3 on thesilicon dioxide film 2 before the ion injection of the conductiveimpurity, and illustrates dependence of peak energy on the filmthickness of the silicon dioxide film 2. Accordingly, the sensitivitycould be regulated through the film thickness of the silicon dioxidefilm 2.

EXAMPLE 9 Evaluation of Activation Ratio of Conductive Impurity inSemiconductor According to an Embodiment of the Present Invention

FIG. 16(b) shows the result of the measurement of the intensity of thesecondary ions measured under the condition where the electrical chargeup did not occur on the surface of the poly-silicon film 3, that is, atan angle of incidence θ=37°, for example, for the sample produced underthe same condition as that of the sample shown in FIG. 16(a). However,the axis of abscissa represents the irradiation time of the primaryions. The reason why this irradiation time does not coincide with theirradiation time on the abscissa of FIG. 16(a) is because the etchingrate differs with the angle of incidence.

By the way, since the primary ions were irradiated under the conditionwhere electrical charge up did not occur in the poly-silicon film 3, theshift of the energy distribution of the intensity of the secondary ionsdid not occur. Accordingly, since the peak value of the intensity of thesecondary ions could always be measured, the intensity of the secondaryions corresponded to the boron quantity in the poly-silicon film 3.

It can be understood from FIG. 16(b) that boron is distributed in asubstantially constant concentration in the poly-silicon film 3. Theconcentration distribution in the depth-wise direction could be obtainedby converting the intensity of the secondary ions to the boronconcentration using the standard sample.

The activation ratio could be evaluated by comparing the carrierconcentration shown in FIG. 16(a) with the boron concentration shown inFIG. 16(b).

We claim:
 1. A measurement method for concentration analysis of a traceelement existing in a solid in a planar direction and/or a depth-wisedirection, comprising:irradiating an ion containing an alkali metalelement to the surface of said solid as an ion beam; detecting athree-component atom composite ion comprising said alkali metal ionirradiated, an object element and a base material element from amongparticles emitted from the surface of said solid due to sputtering bymass separation; and measuring a concentration distribution in theplanar direction by displaying an intensity distribution of saidthree-atom composite ion as a two-dimensional image, or measuring theconcentration distribution in the depth-wise direction by displaying theintensity of said three-atom composite ion with a sputtering time.
 2. Ameasurement method according to claim 1, wherein an angle of incidenceof said irradiation ion beam is set to from 30° to 90° with respect tothe normal of the surface of the solid, in order to improve thesensitivity.
 3. A measurement method according to claim 1, wherein anangle of incidence of said irradiation ion beam is set to from 60° to90° with respect to the normal of the surface of the solid, in order toimprove the resolution in the depth-wise direction.
 4. A measurementmethod according to claim 1, wherein an angle of incidence of saidirradiation ion beam is set to from 0° to 30° with respect to the normalof the sample, in order to improve resolution in the planar direction.5. A measurement method according to claim 1, wherein an angle ofincidence of said irradiation alkali metal ion beam is set to from 60°to 90° to a solid sample having an oxide film on the surface or betweenlayers thereof, in order to make a high-sensitivity analysis of a traceelement contained in said oxide film.